Fix C++20 build resulting in deprecated implicit copy assignment operator warning

include/spdlog/details/log_msg.h

@@ -15,6 +15,7 @@ struct SPDLOG_API log_msg
     log_msg(source_loc loc, string_view_t logger_name, level::level_enum lvl, string_view_t msg);
     log_msg(string_view_t logger_name, level::level_enum lvl, string_view_t msg);
     log_msg(const log_msg &other) = default;
+    log_msg &operator=(const log_msg &other) = default;
 
     string_view_t logger_name;
     level::level_enum level{level::off};